Multi-channel WDM systems are used in order to enhance the transmission capability of optical fiber networks, so that channels which previously would have to be transmitted on a plurality of separate fiber pairs now can be forwarded on a single fiber pair. Using optical wavelength division multiplexed channels means that a plurality of serial information signals, i.e. a plurality of serial binary signals, are transmitted on the same optical fiber by modulating such a serial signal on a light signal having a definite wavelength and then combining the modulated light signals in an optical coupler or optical multiplexer to a composite light signal on the considered optical fiber. The signal primarily modulated on a monochromatic carrier light signal together with the carrier can be called a channel or traffic channel.
Optical wavelength multiplexing can be used to construct different optical fiber network solutions, e.g. solutions using point to point links, networks using a "full mesh bus" or a hubbed bus, etc. All bus solutions require the possibility to add and drop one or more wavelength channels, see FIG. 1, at selected places of the fiber optical network, these places being called add/drop nodes. For the fiber optical WDM network of FIG. 1 having two add/drop nodes, a left line cable 1 comprising two optical fibers 3, 5 for transmission of light signals in the two opposite directions are coupled to one side of such an optical add/drop node 7, the other side of the node 7 being connected to the two fibers 9, 11 of a right line cable 13. The node 7 is connected to or contains receivers 15 and transmitters 17 for converting optical signals to electrical signals and vice versa, the electrical signals being transferred or received respectively from other devices, links or networks, not shown. The line cables 1, 13 ending at a node 7 have their other ends connected to other, for example identically constructed nodes 7. The add and drop operation of such a node can be executed using blocking or non-blocking drop operation. A blocking drop operation means that all the information of the dropped optical wavelength is filtered out and blocked in the add/drop node. A non-blocking drop operation means that only a portion of the power of the dropped optical wavelength is filtered out. The rest of the signal power of the considered wavelength proceeds through the network, through the node and on to the line cable connected to the opposite side of the node.
One important requirement of such an optical add/drop node is that the node should add as little noise as possible, e.g. noise produced by amplified spontaneous emission (ASE) in fiber optical amplifiers usually included in the node, and that the node should have a flat frequency transfer function for light transferred from one side to the opposite side thereof and a balance between the power of added and passing wavelength channels on an optical output line of the node, i.e. the optical output power should be at least approximately the same for every wavelength channel.
One prior art design of an add/drop node having a blocking drop operation is shown in FIG. 2. The optical WDM traffic enters the node through an optional optical preamplifier 19, 21. Each wavelength is filtered out in a demultiplexing element DEMUX 23, 25 separating the channels so that on each of the plurality of output fibers of the demultiplexer 23, 25 only one channel is transmitted, i.e. information carried by light within a single wavelength interval. Dropped channels are fed to receivers 15 through optical fibers 27, 29, each such fiber extending from a demultiplexer 23, 25 to a combining coupler 30 connected to the input terminal of a receiver for the dropped wavelength. Channels to be added as well as passing channels are fed to a multiplexing element MUX 31, 33. Its corresponding input terminals are connected through optical fibers 35, 37, each such fiber extending to a multiplexer 31, 33 from a splitting coupler 38, which is connected to the output terminal of a transmitter 17 for the wavelength to be added. The other input terminals of a multiplexer 31, 33 are connected through optical fibers 39, 41 to respective output terminals of the respective demultiplexer 23, 25 for the same transmission direction through the node. The WDM traffic at the output of the MUX 31, 33 is fed to an optional optical power or booster amplifier 43, 45 and therefrom to the respective fiber of the opposite line cable 1, 13.
The drawback of the solution illustrated in FIG. 2 is that it implies a relatively large number of cascaded filters, i.e. the in-line demultiplexers 23, 25 and in-line multiplexers 31, 33, if a large number of add/drop nodes 7 exist in the network. An additional drawback is that a non-blocking drop operation can not be implemented.
In the published International patent application WO 97/06616 an optical wavelength division multiplexing system including branching units is disclosed. A branching unit comprises typically two circulators and a Bragg grating filter connected therebetween. In the optical fiber line carrying the signal to be added an optical amplifier is connected amplifying the signal to be added to a suitable power level controlled in accordance with the power level of a dropped signal. In the published European patent application 0,668,674 a wavelength division multiplexed network system is disclosed having a plurality of nodes connected to form a ring configuration of the self-healing type. The nodes can include couplers, amplifiers and demultiplexers/multiplexers.